The present invention relates to lamp holders and, more particularly, to an arc discharge lamp holder that compensates for movement of the arc within the arc lamp due to the effect of gravity.
In many applications, it is desired to have an electromagnetic radiation source that radiates electromagnetic waves on a particular area. The electromagnetic radiation may be visible for direct observation, or the electromagnetic radiation may be in the non-visible spectrum, such as infrared. In the visible spectrum (light), such applications include sports stadiums, roadways, commercial interiors, and industrial applications, while in the non-visible spectrum, applications include police, military and other surveillance applications.
High intensity arc discharge lamps are commonly used for such applications. In arc discharge lamps, the electromagnetic radiation is derived from a plasma arc formed within an electromagnetic transmissive envelope or arc tube. One form of an arc discharge lamp that is currently employed is a metal halide lamp. In such lamps, the arc is ignited between two electrodes placed at opposite ends of the electromagnetic transmissive envelope. These electrodes define an envelope axis and are connected to a power supply which supplies the proper voltage and current for starting and operating the lamp.
Most arc discharge lamps also include a parabolic reflecting surface. The parabolic reflecting surface has a predetermined focal point or, if the reflecting surface has a longitudinal length, a focal axis. Commonly, the electromagnetic transmissive envelope is placed adjacent the reflecting surface with the envelope axis located on the focal axis in order to reflect electromagnetic radiation in any preselected direction.
Placement of the envelope axis on the focal axis of a parabolic reflecting surface, however, does not insure that the arc within the electromagnetic transmissive envelope, will remain on the focal axis.
When an arc is ignited within an electromagnetic transmissive envelope, the arc does not form on the envelope axis which is a straight line. In fact, the arc forms along an arched path above the envelope axis as it moves or displaces in a direction opposite from the earth's gravitational field. Since the envelope axis normally has been located on the focal axis, the arc arches off the focal axis. The net effect is that the lamp is not then optimally adjusted.
Although the arc will typically be located above the envelope axis, in relation to the reflector, the arc may be located closer to or farther away from the reflecting surface depending on the rotation of the reflecting surface about the focal axis. For example, if there is a desire to reflect electromagnetic radiation upward, the reflecting surface is positioned below the focal axis. With arc movement or displacement away from the earth's gravitational field, the arc is located above the focal axis at a position that is farther away from the reflecting surface than the focal axis. If, however, there is a desire to reflect electromagnetic radiation downward, then the reflecting surface is positioned above the focal axis. Upward arc displacement again causes the arc to be positioned above the focal axis but, in this situation, the arc is at a position that is closer to the reflecting surface than the focal axis. Therefore, the problem of mounting the arc lamp in a lamp holder is not merely to compensate for upward displacement of the arc within the envelope, but rather to provide a lamp holder that compensates for upward arc displacement given any orientation of the reflecting surface relative to that upward arc displacement.